The present invention constitutes an improvement in the strain gauge art. As is known, strain gauges are used for measuring strains in structures of all types. For example, strain gauges are used as transducers in weighing systems or in numerous other well-known strain measuring systems.
The most significant problem today relating to the strain gauge art is the time-consuming and critical application of the strain gauge to the structure whose strain is to be measured. Strain gauges generally include a metal or the like deposited on a film of bakelite or the like. The metal deposit includes a metal element to which electrical leads are soldered.
A typical way in which strain gauges are applied to a member whose strain is to be measured involves cleaning a surface of the member and then adhesively applying a strain gauge to the member. To completely adhesively secure all parts of the strain gauge to the member, pressure is applied to the strain gauge and preferably throughout its surface area to secure all of the surface area of the strain gauge to the member. After the strain gauge has been adhesively secured to the member, suitable electrical leads are connected to the metal element and the electrical leads and strain gauges are then sealed by suitable moisture and air protective material which is attached to the member and surrounds the strain gauge. Since the strain gauge is extremely small, as are the leads, delicate handling is necessary. Also, it is important that the gauge is adhered throughout to the surface of the member.
The present invention specifically provides for an improvement in the application of a strain gauge to a structure whose strain is to be measured. The present invention minimizes the time-consuming, delicate and critical application of the strain gauge to the member.
In general, the present invention solves the aforementioned problem by providing a preassembled strain gauge structure. In accordance with the present invention, the structure includes a strain gauge fixedly secured in a housing and connected with electrical connections which extend through the housing. In order to apply the structure to a member whose strain is to be measured, all that it is necessary to do is to apply adhesive to the strain gauge and fasten the housing unit to the member whose strain is to be measured. Accordingly, the time-consuming problem of separately securing the strain gauge to the member, allowing the adhesive to cure, and then applying the electrical connections to the strain gauge and subsequently enclosing the strain gauge and electrical connections in moisture-tight material are eliminated. In fact, in accordance with the present invention, a strain gauge is applied in, what could be termed, a one-step procedure, namely, the step of applying a preassembled strain gauge unit to the member whose strain is to be measured.
To applicant's knowledge, no one has developed a preassembled strain gauge structure which may be applied as a unit to a member whose strain is to be measured. Applicant, however, recognizes that there are strain gauge probes, such as disclosed in U.S. Pat. No. 3,416,012, which for test purposes are manually applied and manually maintained in engagement with a strain structure during the strain measuring period, and thereafter are removed therefrom. Such structures measure a change in length between two discrete points, and are for individual test purposes and are not unitized with the structure being tested and used for testing over long periods of time as a permanent part of an installed strain measuring system, such as for weighing systems, etc. Applicant further recognizes that there are certain structures which apply a biasing force to a strain sensing element after the strain sensing element is applied to the member whose strain is to be measured. A typical example of such is shown in U.S. Pat. No. 3,433,699. However, to applicant's knowledge, no one has previously developed or even suggested a strain gauge structure for permanent or semipermanent installation and which is preassembled and which may be applied to the member whose strain is to be measured in the simplified and time-shortened manner of the present invention.